The present invention relates generally to magnetic resonance imaging (MRI) devices, and more particularly to MRI devices with at least one shaping coil to shape a magnetic field for imaging a volume, and MRI devices with a single unit support structure.
MRI devices are widely used in the medical community as a diagnostic tool for imaging items such as tissue and organ structures. A conventional MRI device is described, for example, in U.S. Pat. No. 6,172,588 (the “'588 patent” hereafter), which is incorporated by reference herein in its entirety. As described in the '588 patent, known resistive and superconductive (“SC” hereafter) MRI magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped resistive or SC coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart resistive or SC main coils each carrying a large, identical electric current in the same direction. The main coils are thus designed to create a magnetic field of high uniformity within a typically spherical imaging volume centered within the magnet's bore where the object to be imaged is placed.
A single, tubular-shaped shielding assembly may also be used to prevent the high magnetic field created by and surrounding the main coils from adversely interacting with electronic equipment in the vicinity of the magnet. Such shielding assembly typically includes several radially-aligned and longitudinally spaced-apart resistive or SC bucking coils carrying electric currents of generally equal amperage, but in an opposite direction to the electric current carried in the main coils and positioned radially outward of the main coils.
Open magnets, including “C” shaped magnets, typically employ two spaced-apart coil assemblies with the space between the assemblies containing the imaging volume. The spaced-apart coil assemblies thus allow for access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. The patient may be positioned in that space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design.
Known open magnet designs having shielding include those wherein each coil assembly has an open bore and contains a resistive or SC shielding coil positioned longitudinally and radially outward from the resistive or SC main coil(s). It is also known in open magnet designs to place an iron pole piece in the bore of a resistive or SC coil assembly. The iron pole piece enhances the strength of the magnetic field and, by shaping the surface of the pole piece, magnetically shims the magnet improving the homogeneity of the magnetic field. An iron return path is used to connect the two iron pole pieces. It is noted that the iron pole piece also acts to shield the magnet.
Conventional MRI devices, however, have limited imaging accuracy for several reasons. At least part of the limited accuracy is due to manufacturing tolerances, which inherently lead to variations from one manufactured MRI device to another. Manufacturing tolerance errors in known MRI devices are aggravated by conventional multi-unit support structures therein, which may include a considerable number of locations where manufacturing induced variations can occur (e.g., placement and/or alignment errors in positioning coils, etc.).
Furthermore, the MRI magnet itself may suffer from manufacturing variations, such as slight variations in the positioning of the windings thereof, etc. Hence, to compensate for these problems, conventional techniques for manufacturing MRI devices involve intricate and involved regimes for placing and adjusting the MRI device components mounted on the support structure. This can lead to higher manufacturing costs, longer manufacturing lead times, and generally more complex MRI devices.
Thus, a need exists for a MRI device which provides greater magnetic field homogeneity within the magnet's imaging volume, and a method of manufacturing the MRI device to tighter tolerances.